1. Field of the Invention
The present invention relates to an image compression apparatus, method, and program, and more specifically to an image compression apparatus for compressing image data of moving picture or still image to a size smaller than the original data size. The invention also relates to an image expansion apparatus, method, and program for restoring image data compressed by the image compression apparatus.
2. Description of the Related Art
Generally, in the signal compression or encoding techniques, the compressed or encoded data size and quality of a signal obtained by restoring the compressed or encoded data are in a trade-off relationship. That is, if you try to reduce the data size, the signal quality will be degraded greatly, while if you try to avoid signal quality degradation, the data size will become large. In the signal compression or encoding, it is desirable that the signal data size after compression is small while the original signal quality is maintained as much as possible.
U.S. Patent Application Publication No. 20090016426 (Patent Document 1) describes an encoding scheme using a differential signal for encoding an input signal. In Patent Document 1, the encoding apparatus reduces the data size by downsampling an input signal and encodes the reduced input signal to generate first encoded information. Then, the first encoded information is decoded and the decoded data are upsampled to take a difference between the upsampled data and original input signal. Then, the encoding apparatus encodes a differential signal generated by taking the difference to generate second encoded information and transmits the first and second encoded information to a transmission line.
The decoding apparatus in Patent Document 1 receives the first and second encoded information via the transmission line. The decoding apparatus generates first decoded information by decoding the first encoded information and upsamples the first decoded information. Further, the decoding apparatus generates second decoded information (differential signal) by decoding the second encoded information. Then, the apparatus adds the upsampled first decoded information to the second decoded information and outputs the added signal as a regenerated signal corresponding to the input signal.
Here, encoding apparatuses, in general, have unique characteristics that cause quality degradation of decoded signals. In Patent Document 1, impulse response for adjustment is learned using input signals for leaning and impulse response for adjustment is convoluted in the upsampled decoded signal in order to cope with this problem. More specifically, the impulse response for adjustment is convoluted in the decoded information of the upsampled first encoded information in the encoding apparatus. Then, a difference between the signal convoluted with the impulse response for adjustment and the original input signal is calculated and the differential signal is encoded to generate second encoded information. In the decoding apparatus, the impulse response for adjustment is convoluted in the upsampled first decoded information and the second decoded information is added thereto. In doing so, Patent Document describes that the characteristics unique to the encoding apparatus may be cancelled out.
In Patent Document 1, a uniquely appearing defect or noise is learned. By convoluting the impulse response for adjustment in the upsampled decoded signal, a uniquely appearing defect or noise as a certain frequency characteristic may be corrected. Generally, however, the phase between compression noise and image is not constant and the impulse response for adjustment obtained by learning may become a coarsely classified pattern. Consequently, the difference between the signal convoluted with the impulse response for adjustment and the original input signal does not become sufficiently small in general image information. Further, in the case where data is reduced to a small size after compression, noise remains in a generally important object, such as a character or a person's face, having characteristics, such as having detailed image information and quick in motion, resulting inappropriate information for use.
In view of the circumstances described above, it is an object of the present invention to provide an image compression apparatus, method, and program capable of compressing an input image that includes a predetermined target object at a high compression ratio while maintaining a high image quality. It is a further object of the present invention to provide an image expansion apparatus, method, and program for expanding data compressed by the image compression apparatus of the present invention.